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<h2 class="title">
<a id="modules-scripting-engine"></a>Advanced scripts using script engines<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch/edit/7.7/docs/reference/scripting/engine.asciidoc">edit</a>
</h2>
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<p>A <code class="literal">ScriptEngine</code> is a backend for implementing a scripting language. It may also
be used to write scripts that need to use advanced internals of scripting. For example,
a script that wants to use term frequencies while scoring.</p>
<p>The plugin <a href="/guide/en/elasticsearch/plugins/7.7/plugin-authors.html" class="ulink" target="_top">documentation</a> has more information on
how to write a plugin so that Elasticsearch will properly load it. To register
the <code class="literal">ScriptEngine</code>, your plugin should implement the <code class="literal">ScriptPlugin</code> interface
and override the <code class="literal">getScriptEngine(Settings settings)</code> method.</p>
<p>The following is an example of a custom <code class="literal">ScriptEngine</code> which uses the language
name <code class="literal">expert_scripts</code>. It implements a single script called <code class="literal">pure_df</code> which
may be used as a search script to override each document’s score as
the document frequency of a provided term.</p>
<div class="pre_wrapper lang-java">
<pre class="programlisting prettyprint lang-java">private static class MyExpertScriptEngine implements ScriptEngine {
    @Override
    public String getType() {
        return "expert_scripts";
    }

    @Override
    public &lt;T&gt; T compile(
        String scriptName,
        String scriptSource,
        ScriptContext&lt;T&gt; context,
        Map&lt;String, String&gt; params
    ) {
        if (context.equals(ScoreScript.CONTEXT) == false) {
            throw new IllegalArgumentException(getType()
                    + " scripts cannot be used for context ["
                    + context.name + "]");
        }
        // we use the script "source" as the script identifier
        if ("pure_df".equals(scriptSource)) {
            ScoreScript.Factory factory = new PureDfFactory();
            return context.factoryClazz.cast(factory);
        }
        throw new IllegalArgumentException("Unknown script name "
                + scriptSource);
    }

    @Override
    public void close() {
        // optionally close resources
    }

    @Override
    public Set&lt;ScriptContext&lt;?&gt;&gt; getSupportedContexts() {
        return Collections.singleton(ScoreScript.CONTEXT);
    }

    private static class PureDfFactory implements ScoreScript.Factory,
                                                  ScriptFactory {
        @Override
        public boolean isResultDeterministic() {
            // PureDfLeafFactory only uses deterministic APIs, this
            // implies the results are cacheable.
            return true;
        }

        @Override
        public LeafFactory newFactory(
            Map&lt;String, Object&gt; params,
            SearchLookup lookup
        ) {
            return new PureDfLeafFactory(params, lookup);
        }
    }

    private static class PureDfLeafFactory implements LeafFactory {
        private final Map&lt;String, Object&gt; params;
        private final SearchLookup lookup;
        private final String field;
        private final String term;

        private PureDfLeafFactory(
                    Map&lt;String, Object&gt; params, SearchLookup lookup) {
            if (params.containsKey("field") == false) {
                throw new IllegalArgumentException(
                        "Missing parameter [field]");
            }
            if (params.containsKey("term") == false) {
                throw new IllegalArgumentException(
                        "Missing parameter [term]");
            }
            this.params = params;
            this.lookup = lookup;
            field = params.get("field").toString();
            term = params.get("term").toString();
        }

        @Override
        public boolean needs_score() {
            return false;  // Return true if the script needs the score
        }

        @Override
        public ScoreScript newInstance(LeafReaderContext context)
                throws IOException {
            PostingsEnum postings = context.reader().postings(
                    new Term(field, term));
            if (postings == null) {
                /*
                 * the field and/or term don't exist in this segment,
                 * so always return 0
                 */
                return new ScoreScript(params, lookup, context) {
                    @Override
                    public double execute(
                        ExplanationHolder explanation
                    ) {
                        return 0.0d;
                    }
                };
            }
            return new ScoreScript(params, lookup, context) {
                int currentDocid = -1;
                @Override
                public void setDocument(int docid) {
                    /*
                     * advance has undefined behavior calling with
                     * a docid &lt;= its current docid
                     */
                    if (postings.docID() &lt; docid) {
                        try {
                            postings.advance(docid);
                        } catch (IOException e) {
                            throw new UncheckedIOException(e);
                        }
                    }
                    currentDocid = docid;
                }
                @Override
                public double execute(ExplanationHolder explanation) {
                    if (postings.docID() != currentDocid) {
                        /*
                         * advance moved past the current doc, so this
                         * doc has no occurrences of the term
                         */
                        return 0.0d;
                    }
                    try {
                        return postings.freq();
                    } catch (IOException e) {
                        throw new UncheckedIOException(e);
                    }
                }
            };
        }
    }
}</pre>
</div>
<p>You can execute the script by specifying its <code class="literal">lang</code> as <code class="literal">expert_scripts</code>, and the name
of the script as the script source:</p>
<div class="pre_wrapper lang-console">
<pre class="programlisting prettyprint lang-console">POST /_search
{
  "query": {
    "function_score": {
      "query": {
        "match": {
          "body": "foo"
        }
      },
      "functions": [
        {
          "script_score": {
            "script": {
                "source": "pure_df",
                "lang" : "expert_scripts",
                "params": {
                    "field": "body",
                    "term": "foo"
                }
            }
          }
        }
      ]
    }
  }
}</pre>
</div>
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